Department of Physics, Chemistry and Biology
Master’s Thesis
Design and construction of ultrahigh vacuum system to fabricate
Spintronic devices, fabrication and characterization of
OMAR (organic magnetoresistance) devices
Srikrishna Chanakya Bodepudi
LITH-IFM-A-EX--09/2189—SE
12th June 2009
Department of Physics, Chemistry and Biology Linköpings University
Presentation Date
12th June 2009
Publishing Date (Electronic version)
Department and Division Surface physics and chemistry, IFM
URL, Electronic Version
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-2189
Publication Title: Design and construction of ultrahigh vacuum system to fabricate
Spintronic devices, fabrication and characterization of OMAR (organic magnetoresistance) devices
Author(s): Srikrishna Chanakya Bodepudi
Abstract
This thesis concerns design and construction of an ultra high vacuum chamber to fabricate and characterize spintronic devices. The long term intention is to fabricate spin valve structures with V[TCNE]2 (hybrid organic inorganic
semiconductor room temperature magnet) sandwiched between two ferromagnetic electrodes, which requires better than 10-8mbar of vacuum. Due to an uncured leak in the chamber, the current vacuum is limited to 4*10-7mbar. The V[TCNE]2
thin film prepared in this vacuum, oxidized completely by the presence of oxygen during the film growth. Organic magnetoresistance (OMAR) devices which are simple organic diode structures were fabricated and characterized, as they are compatible with high vacuum conditions. A magnetoresistance measurement set up was arranged and the possible problems in fabrication and characterization are analyzed.
To fabricate OMAR devices-ITO/P3HT/Al, RR-P3HT (regio regular poly (3-hexylthiophene)) an effective hole transport polymer with higher hole mobilities was used as an active layer and Al (aluminum) as a cathode. A thermal evaporation setup was added to the vacuum chamber to evaporate Al electrodes. The devices were kept in argon and vacuum environments, while characterizing in dark to suppress the exitons generated by photo illumination. The Organic magnetoconductance of about 1% is observed for the less concentration P3HT (3mg/1ml), and significantly improved to -23% for the high concentration P3HT (10mg/ml) solution. The results support that the negative magnetoconductance is due to the formation of bipolaron under the influence of an external magnetic field.
Finally, suggestions to improve the performance of the vacuum chamber to fabricate and characterize the spintronic devices and OMAR devices are presented.
Keywords
Ultra high vacuum, Spintronics, Organic spintronics, Magnetoresistance, GMR, OMAR (Organic magnetoresistance), V[TCNE]2, Hybrid organic/inorganic magnet.
Language
X English
Other (specify below)
Number of Pages 56 Type of Publication Licentiate thesis X Degree thesis Thesis C-level Thesis D-level Report
Other (specify below)
ISBN (Licentiate thesis)
ISRN:
LITH-IFM-A-EX--09/2189--SE
Title of series (Licentiate thesis) Series number/ISSN (Licentiate thesis)Department of physics, chemistry and biology
LITH-IFM-A-EX--09/2189—SE 12th June 2009
Design and construction of ultrahigh vacuum system to fabricate
Spintronic devices, fabrication and characterization of
OMAR (organic magnetoresistance) devices
Carried out in Surface Physics and Chemistry Laboratories at IFM, LiU
Srikrishna Chanakya Bodepudi
Supervisor and examiner
Prof. Mats Fahlman
Abstract
This thesis concerns design and construction of an ultra high vacuum chamber to fabricate and characterize spintronic devices. The long term intention is to fabricate spin valve structures with V[TCNE]2 (hybrid organic inorganic semiconductor room temperature magnet) sandwiched between two ferromagnetic electrodes, which requires better than 10 -8
mbar of vacuum. Due to an uncured leak in the chamber, the current vacuum is limited to 4*10-7mbar. The V[TCNE]2 thin film prepared in this vacuum, oxidized completely by the presence of oxygen during the film growth. Organic magnetoresistance (OMAR) devices which are simple organic diode structures were fabricated and characterized, as they are compatible with high vacuum conditions. A magnetoresistance measurement set up was arranged and the possible problems in fabrication and characterization are analyzed.
To fabricate OMAR devices-ITO/P3HT/Al, RR-P3HT (regio regular poly (3-hexylthiophene)) an effective hole transport polymer with higher hole mobilities was used as an active layer and Al (aluminum) as a cathode. A thermal evaporation setup was added to the vacuum chamber to evaporate Al electrodes. The devices were kept in argon and vacuum environments, while characterizing in dark to suppress the exitons generated by photo illumination. The Organic magnetoconductance of about 1% is observed for the less concentration P3HT (3mg/1ml), and significantly improved to -23% for the high concentration P3HT (10mg/ml) solution. The results support that the negative magnetoconductance is due to the formation of bipolaron under the influence of an external magnetic field.
Finally, suggestions to improve the performance of the vacuum chamber to fabricate and characterize the spintronic devices and OMAR devices are presented.
Acknowledgement
First, I would like to thank my supervisor and examiner Prof. Mats Fahlman for giving this opportunity and all his help and patience during this thesis.
I have put a lot of work and time into my attempt to construct vacuum chamber to fabricate spintronic devices. This could not have been possible without kind assistance of some people to whom I would like to express my gratitude.
Anders Evaldsson has my sincere gratitude for his help with the equipment and Per Erlandsson for helping me in lab view. I am always thankful to the group members, Surface Physics and Chemistry for valuable discussions and who never hesitated to help in my work. Last, but not least, I would like to thank my family and friends for their support and motivation.
Table of Contents
1.INTRODUCTION………1
1.1. Organic- based spintronics………..2
1.2. Organic –based magnets………..4
1.3. RR-P3HT (region regular poly(hexylthiophene))………..7
1.4. Ultra high vacuum equipment………..8
2.THEORY……….10
2.1. Physical concepts relevant to spintronic devices………..10
2.1.1.Spin-orbit coupling………..12
2.1.2.Hyperfine interaction……….14
2.2. Ferromagnetic half metals………..………..16
2.3. Magnetoresistance………..19
2.3.1.Giant magnetoresistance(GMR)………..19
2.3.2.Tunnel magnetoresistance (TMR)………..21
2.3.3.Colossal magnetoresistance(CMR)………...21
2.4. Organic magnetoresistance (OMAR)………..22
2.5. Chemical vapor deposition (CVD)……….26
3.EXPERIMENTAL………28
3.1. Laboratory equipment………..28
3.2. Spin coating………..28
3.3. Vacuum chamber……….28
3.4. Masks to pattern electrodes for spintronic devices……….29
3.5. Helium leak testing……….30
3.6. Evaporation source for metal electrodes………32
3.7. Magnetoresistance measurement setup………...34
4.FABRICATION……… ……….36
4.1 V[TCNE]2 thin film growth………..36
4.2 Fabrication of OMAR devices………37
4.2.1 Sample preparation………..37
4.2.2 OMAR device characterization………...38
5 DISCUSSION ……….43
6 CONCLUSION ………..44
7 RECOMMENDATIONS……….44
1
1 Introduction
In conventional electronics, charge transport that involves motion of charged particles in the material is used to convey information. In Spintronics, spin degrees of freedom is added as well. Although spintronics emerged as a major field of research after the discovery of giant magnetoresistance (GMR) effect, the initiation of spintronics is the observation of anisotropic magnetoresistance- a few percent variations in resistance on relative orientation of magnetization and current, by Lord Kelvin in 18561. A detailed explanation of different magnetoresistance effects is given later in the text. Organic semiconducting materials with properties of high spin relaxation time and semiconducting charge transport have enabled a new research area- organic spintronics2. With the Recent developments in low-cost synthetic organic based spintronic materials, many organic spintronic materials are an alternative for the conventional spintronic materials which could significantly decrease the production cost. Besides, the discovery of Organic magnetoresistance (OMAR) effect in Organic light emitting diode (OLED) structures without magnetic electrodes, is an encouraging step to understand the spin based Physics in semiconducting organic materials.
V[TCNE]2, a hybrid organic inorganic thin film room temperature magnet, with
semiconducting and soft magnetic properties, is an option to replace traditional atom based magnets in spintronic applications4,3. Moreover, hybrid organic inorganic magnets are also a cheap alternative to the magnets used in load-speakers, microphones, head phones, etc. However extreme air-sensitiveness and difficulty in reproduction of V[TCNE]2 are the major hurdles that still has to overcome4. At present, V[TCNE]2 thin
films can be produced by chemical vapor deposition (CVD) or Physical vapor deposition (PVD) methods at UHV( Ultra high vacuum) conditions.
In general, achieving required vacuum conditions is the deciding factor for using particular analytical tool or to grow uniform thin films, especially when the precursors or depositing materials are sensitive to atmospheric gases. For the fabrication of spintronic devices using ferromagnetic electrodes and hybrid organic inorganic magnets, ultra high vacuum condition better than 10-8 mbar is necessary. In special cases, for instance spin valves using V [TCNE]2 as an interfacial layer, the device requires in situ characterization
due to the extreme air-sensitiveness of V[TCNE]2 films5. Whereas for OMAR devices
with organic small molecules or conjugated polymers as active layers which are relatively resistive to atmospheric gases, high vacuum(HV) conditions are sufficient for the device fabrication with Ca, Al or Au as electrodes.
In this thesis, the pre-existing vacu
the fabrication of organic spintronic devices, organic magnetoresistance (
the thin film deposition is
simple schematic representation of vacuum requirements for the deposition methods thin films relevant to this thesis are
Figure.1 Vacuum requirements to develop thin films for spintronic devices.
1.1 Organic-based spintronics:
With unique mechanical, optical and electronic properties, organic semi materials (OSC) have already made significant contribution in developing
cheap electronic devices. Still there are many interesting properties in organic semiconducting materials not given much attention in the present scientific world. But the rise of scientific interest in spintronics, researchers have been sear
materials to invent new physical phenomena in spintronics or to improve the functionality of the spintronic devices25
interfacial layer in spin valves, which magnetically de
2
existing vacuum chamber is redesigned and constructed to facilitate the fabrication of organic spintronic devices, from hybrid organic/inorganic spin valves to magnetoresistance (OMAR) devices. Achieving required vacuum conditions for is an important step to fabricate above mentioned devices. A simple schematic representation of vacuum requirements for the deposition methods
nt to this thesis are depicted in Fig1.
Vacuum requirements to develop thin films for spintronic devices.
based spintronics:
unique mechanical, optical and electronic properties, organic semi materials (OSC) have already made significant contribution in developing
cheap electronic devices. Still there are many interesting properties in organic semiconducting materials not given much attention in the present scientific world. But the rise of scientific interest in spintronics, researchers have been sear
materials to invent new physical phenomena in spintronics or to improve the functionality
25
. One of the major challenges is to find suitable material as an interfacial layer in spin valves, which magnetically decouples the two ferromagnetic um chamber is redesigned and constructed to facilitate ganic/inorganic spin valves to required vacuum conditions for above mentioned devices. A simple schematic representation of vacuum requirements for the deposition methods of
Vacuum requirements to develop thin films for spintronic devices.
unique mechanical, optical and electronic properties, organic semi-conducting materials (OSC) have already made significant contribution in developing flexible and cheap electronic devices. Still there are many interesting properties in organic semiconducting materials not given much attention in the present scientific world. But the rise of scientific interest in spintronics, researchers have been searching for suitable materials to invent new physical phenomena in spintronics or to improve the functionality is to find suitable material as an couples the two ferromagnetic
3
electrodes and allows spin-coherent charge transportation through it. Spin valves currently in use are of metal or metal-insulator structures with less spin coherent charge transport and lack of amplification. Recently, semiconducting materials with magnetic doping are used in spintronic devices for amplification to increase the spin-relaxation length (Ls) - The length a charge can travel without flipping its spin. However, the
spin-orbit coupling and hyperfine interaction which are the vital driving forces to flip spin get stronger with increasing atomic number (Z)24. Consequently in semiconducting materials, high Ls are not possible unless using magnetic dopants which are an expensive
alternative to the currently using metal-insulator structures. At this point, once again the organic semiconducting materials came into picture with atoms of low Z (carbon and hydrogen) which provide long Ls.
Even though the research in developing pure organic magnets is effectively going on, the semiconducting and spin based charge transport are the elementary properties to place organic semiconducting materials as a suitable alternative interfacial layer in spin valves2. Besides that, the discovery of organic magnetoresistance (OMAR) effect without ferromagnetic electrodes (no spin dependent charge injection and detection) opened a better alternative to understand the spin based device physics of organic semiconducting materials. However, to understand the spin degrees of freedom in organic semiconducting materials, it is required to begin with its charge transport2.
In most of the organic semiconductors charge transport is by hopping26. On addition or removal of an electron from the organic semi-conducting material, there is a redistribution of charge to minimize energy, resulting in changes in bond-lengths, bond angles and nuclear positions. This change in lattice configuration around charge is termed as polaron. It reduces the overall energy of the system and new energy levels appear with in the HOMO (highest occupied molecular orbital) - LUMO (lowest unoccupied molecular orbital) energy gap(Fig.2). A polaron carries spin half, whereas two nearby polarons in a single unit referred as bipolaron is spinless26.
Figure. 2 Representation of new energy levels within the energy gap by the formation of polaron
4
When two polarons of opposite charge bound together by columbic interaction, and close enough to define in a single wave function can be defined as an exiton. Columbic binding lowers energy and drops exiton into energy gap. An exiton can also be generated by absorbing a photon of energy equal or greater than exiton energy, by lifting an electron from HOMO level and placing in LUMO level26(Fig.3).
Figure. 3 Formation of exiton by the combination of electron and hole polarons.
1.2 Organic based magnets:
In contrast to the conventional magnets, organic magnets can be prepared by solution or vapor phase deposition methods with the knowledge of organic chemistry and physical chemistry instead of ceramic or metallurgical engineering techniques. The contribution of organic units in organic based magnets is to provide unpaired electron spins for magnetic ordering in the material. The spins provided by the organic units are related with the s, p and п orbital, in spite of d or f orbital electrons as in transition metals27. M(TCNE)x,
where M= V, Fe, Mn, Co, Ni etc., TCNE= tetracynoethylene, with x=2 is one of the most studied family of organic based magnets(Fig.22b) which are three dimensional (3D) magnets. These systems are first reported in a powder form prepared from the solution and represented as M(TCNE)x.yS where S=solvent: dichloromethane, acetonitrile,
tetrahydrofuran. In particular V(TCNE)x.yS has raised interest over other systems since it
Figure.4 (a) chemical structure of TCNE. (b) Curie temperatures of different M(TCNE)
V(TCNE)x films with improved transition te
sensitivity are obtained by developing solvent free thin films by structure of the V(TCNE)
data analysis , the oxidative state is proposed as V the vanadium ion (VII) and ½ for the [TCNE]
surrounded by up to six ligands and each ligand, [TCNE] different vanadium ions (
V[TCNE]x is a semiconducting room temperature magnet with a conductivity of about
10-4Scm-1. The conductivity decreases with decreasing temperature hopping like charge transpo
soft magnet29.
The best way to understand the ground state electronic structure of V[TCNE] starting with crystal field splitting
due to the presence of ligands forms30,31. The ground state 3d
Figure.5 Frontier electronic structure of V[TCNE]
5
(a) chemical structure of TCNE. (b) Curie temperatures of different M(TCNE)
improved transition temperature(Tc) and to some extent sensitivity are obtained by developing solvent free thin films by CVD. Even though the structure of the V(TCNE)x is not known completely, based on magnetic and elemental
tive state is proposed as VII (x=2). The spin involved are 3/2 for ) and ½ for the [TCNE]- . In V[TCNE]x, x~2 each vanadium is
surrounded by up to six ligands and each ligand, [TCNE]- is binding to up to four different vanadium ions (Fig.4). Unlike transition and rare-earth based metallic magnets, is a semiconducting room temperature magnet with a conductivity of about
The conductivity decreases with decreasing temperature
hopping like charge transportation. A low coercive field of about 4.5Oe is recorded
The best way to understand the ground state electronic structure of V[TCNE] ng with crystal field splitting- a well known property of 3d elements
due to the presence of ligands ([TCNE]-) slightly distorted octagonal environment d state 3d- degenerated orbital splits into triply (t2g
Frontier electronic structure of V[TCNE]2. (a) Ground state. (b) After adding one electron.
Metal (M) Tc(K) V 370 Fe 95 Mn 75 Co 40 Ni 40
(a) chemical structure of TCNE. (b) Curie temperatures of different M(TCNE)x.
mperature(Tc) and to some extent air . Even though the is not known completely, based on magnetic and elemental (x=2). The spin involved are 3/2 for , x~2 each vanadium is is binding to up to four earth based metallic magnets, is a semiconducting room temperature magnet with a conductivity of about The conductivity decreases with decreasing temperature which suggests rtation. A low coercive field of about 4.5Oe is recorded as a
The best way to understand the ground state electronic structure of V[TCNE]x is by
a well known property of 3d elements10. In this case, slightly distorted octagonal environment
2g) and doubly (eg)
(a) Ground state. (b) After adding one electron.
Tc(K) 370 95 75 40 40
degenerated orbitals with certain energy gap (
V[TCNE]2, the highest occupied molecular orbital (HOMO) is mainly localized on V(3d)
units and is characterized by str
the lowest unoccupied molecular orbital (LUMO) is localized on [TCNE] adding an extra electron to this system
(Fig.5a, b).
The CVD based thin-film development of V[TCNE]
improved characterization techniques such as UPS (ultraviolet photoelectron spectroscopy) and XPS (X
growth is a necessary step to utilize V[TCNE] valves. Magnetoresistance measurements of CVD
increase in resistance (positive magnetoresistance) of 0.7% in an applied magnetic field of 600mT. The effect is three orders of magnitude more than the magnetoresistance in conventional disordered semiconductors
was explained by a charge transport hopping model based on the frontier electronic structure of V[TCNE]2.
In this thesis, necessary steps to fabricate spin valves using V[TCNE] layer are studied and required
a spin valve structure by sandwiching V[TCNE] ferromagnetic electrodes. The soft magnetism
provides new functionality in spin valves by switching the interfacial layer instead of switching ferromagnetic electrodes from parallel to anti
if Co electrode is deposited on both sides of V[TCNE]
transportation can be varied by switching the magnetization of V[TCNE] to anti-parallel(Fig.6).
Figure.6 Spin valve with V[TCNE]
electrodes gives low resistance state. (b) Anti
6
degenerated orbitals with certain energy gap (∆) termed as crystal field splitting. In , the highest occupied molecular orbital (HOMO) is mainly localized on V(3d) units and is characterized by strong hybridization between V(3d) and [TCNE]
the lowest unoccupied molecular orbital (LUMO) is localized on [TCNE]
adding an extra electron to this system, creates a coulomb gap around the Fermi level
film development of V[TCNE]2 enables the use of new and
improved characterization techniques such as UPS (ultraviolet photoelectron spectroscopy) and XPS (X-ray photoelectron spectroscopy)27. Moreover, thin
necessary step to utilize V[TCNE]2 in spintronic devices, for instance in spin
valves. Magnetoresistance measurements of CVD-prepared V[TCNE]x films shows increase in resistance (positive magnetoresistance) of 0.7% in an applied magnetic field The effect is three orders of magnitude more than the magnetoresistance in conventional disordered semiconductors27. This anomalously large magnetoresistance was explained by a charge transport hopping model based on the frontier electronic
steps to fabricate spin valves using V[TCNE]
required equipment is arranged. The primary interest is to fabricate a spin valve structure by sandwiching V[TCNE]2 with two different
c electrodes. The soft magnetism and low coercive field of V[TCNE] provides new functionality in spin valves by switching the interfacial layer
instead of switching ferromagnetic electrodes from parallel to anti-paral
if Co electrode is deposited on both sides of V[TCNE]2, the spin coherent charge
transportation can be varied by switching the magnetization of V[TCNE]
Spin valve with V[TCNE]2 as an interfacial layer. (a) Parallel orientation with the ferromagnetic
electrodes gives low resistance state. (b) Anti-parallel orientation gives high resistance state.
) termed as crystal field splitting. In , the highest occupied molecular orbital (HOMO) is mainly localized on V(3d) ong hybridization between V(3d) and [TCNE]- units. And the lowest unoccupied molecular orbital (LUMO) is localized on [TCNE]- units29,30,31. By p around the Fermi level
enables the use of new and improved characterization techniques such as UPS (ultraviolet photoelectron . Moreover, thin-film in spintronic devices, for instance in spin prepared V[TCNE]x films shows increase in resistance (positive magnetoresistance) of 0.7% in an applied magnetic field The effect is three orders of magnitude more than the magnetoresistance in . This anomalously large magnetoresistance was explained by a charge transport hopping model based on the frontier electronic
steps to fabricate spin valves using V[TCNE]2 as an interfacial
rimary interest is to fabricate with two different or similar and low coercive field of V[TCNE]2
provides new functionality in spin valves by switching the interfacial layer- (V[TCNE]2),
parallel. For instance, the spin coherent charge transportation can be varied by switching the magnetization of V[TCNE]2 from parallel
as an interfacial layer. (a) Parallel orientation with the ferromagnetic parallel orientation gives high resistance state.
7
However the major difficulty to fabricate spin valve structure with V[TCNE]2 as
interfacial layer lies in the air sensitivity of this material. So, the ultra high vacuum conditions are necessary to fabricate as well as to characterize the device. Characterization of this spin valve helps to further understand the charge transportation and magnetic properties of V[TCNE]2.
1.3 RR-P3HT (Regio regular polyhexylthiophene)
P3HT is a widely studied polymer and well known as an effective hole transport polymer with higher hole mobilities than most of the semiconducting polymers including poly (phenylenevinylene). Initially P3HT was only available in regio random form which has very poor hole mobilities32. But the hole mobilities are significantly improved with regio regular P3HT (Fig.7). The increase in mobility is due to side-chain–induced self organization into a well-ordered two dimensional lamellar structure32. In this thesis RR-P3HT is used as an active layer for the OMAR (Organic magnetoresistance) devices.
8
1.4 Ultra high vacuum equipment
Vacuum system plays a crucial role in Surface physics. In this thesis, an Ultra high vacuum chamber is used to accommodate thin film deposition. The main reason to use vacuum environment is to avoid the reaction of gases with the sample of interest. Moreover, the experimental probes such as UPS and XPS used to analyze or measure sample properties depends on electron or other beam emissions which could not exist without suitable vacuum.
Atmospheric pressure is usually around 1.0 atm = 1.01*105 Pa = 1.01 bar = 760 Torr and the particle density is roughly 10+25cm-3 which indeed makes very difficult to maintain clean environment. A rough vacuum can be achieved by a simple rotary vane pump to around 10-3 mbar. Usually rubber gaskets are used in this pump. High vacuum in between 10-3 mbar to 10-8 mbar made possible with different classes of pumps, diffusion, turbo molecular and cryogenic pumps. Diffusion pump works by hot oil or mercury with no moving parts and with a backing rotary vane pump (Fig.8). It can be used in molecular beam experiments where contamination is not a serious effect. However, turbo molecular pump is better suitable for contamination free vacuum. It contains mechanical devices with a stack of rotating vanes with blades pitched at different angles to remove residual gas molecules in a single direction. Proper care must be taken when turbo pump is backed up with a rotary vane pump while controlling the speed of the pump.
Ultra high vacuum(UHV) starts from 10-9 down to 10-11 mbar can be achieved with ion pumps. Unlike diffusion pump, ion pump contain no contaminating substances even it has no moving parts like diffusion pump. It uses high voltages for cold cathode emissions to ionize residual atoms or molecules in the chamber and collects by the anode in chemically reactive plates by extremely high electric fields. UHV also possible with turbo pumps by baking the chamber to about 500K to evaporate and pump down the water molecules stick to the walls of the chamber.
9
In this thesis, high vacuum range that is in between 10-6 and 10-8 mbar is used for deposition of V(tcne)2 and metal thin-films. A schematic representation of vacuum
chamber used in this thesis with vacuum pumps is shown in Fig.8. Ion pump is not used as almost all experiments are taken place close to 10-8 mbar.
To measure pressure inside the chamber, hot cathode ionization gauge is generally used. It is a triode with cathode as a filament. In this thesis Bayard-Alpert ionization gauge is used which works with the same principle of triode ionization gauge and made by inverting geometry of the triode tube: putting a small diameter wire in the centre, surrounding the collector with a grid and finally putting the filament outside the grid (Fig.9). By applying regulated electron current of 10mA, the electrons came out of the filament are attracted by the grid under positive potential of 150V. Due to the magnetic field at the centre of the grid, the electrons not impinge immediately to the grid but oscillate in the space inside the grid and ionize the gas molecules. The ionized gas molecules are collected by the ion collector wire at the centre of the grid with a negative potential of -30V. Ion currents which differ for different gases at same pressure are in the order of 1mA/Pa. Since the molecule density of gases is proportional to the pressure, the pressure inside the chamber can be calibrated by the ion current6. Hot cathode filaments damages when exposed to atmospheric pressure or even to low vacuum. It is able to calibrate the pressure range from 10-3mbar to 10-10mbar.
(a) (b)
10
2 Theory
2.1 Physical concepts relevant to spintronic devices:
Magnetism generates in a material due to the proper alignment of magnetic field produced by the orbital angular momentum of the electron and its intrinsic angular momentum (spin). The magnetic field due to the orbital angular momentum, changes with the external magnetic field. When atoms or molecules are exposed to an external permanent magnet or an electromagnet, according to the Lenz’s law, electrons change orbital velocity around the nucleus to screen the external magnetic field10(Fig.10). In the case of diamagnetic materials, the induced magnetization (M) in the material develops opposite to the applied magnetic field (H).
M= χH (2.1) Where χ is called magnetic susceptibility (degree of magnetization (M) of a material in response to an applied magnetic field.) expressed in emu/mol.
Figure.10 Pictorial depiction of Lenz’s law. (Adapted from Prof.F.Joseph’s home page, San Jose
University)
In principle, the charge moves in association with its intrinsic angular momentum, spin of the charge carrier (Fig.10, 11). The quantization of the spin can be represented as,
S = msћ ; where ms = ± ½ for an electron. (2.2)
11
Figure.11 The intrinsic angular momentum (spin ) of the electron.
The magnetic dipole moment by the spin of a charge carrier can be represented as,
µ = -(glml+gsms) eη/(2m) (2.3)
Where ml -orbital angular momentum, ms- intrinsic angular momentum.
The origin of permanent magnetic moment of a molecule depends on the unpaired electron spin. The magnitude of magnetic moment of an electron is proportional to the intrinsic angular momentum (spin) of that electron, [s(s+1] 1/2ћ.
µ = ge[s(s+1] 1/2µb (2.4)
Where µb = Bohr magneton = (e ћ)/ (2me), ge = gyro magnetic ratio of electron=2.0023
If there is more than one electron in a molecule, then the total spin of each molecule can be taken as S. Then s[s+1] should be replaced by S[S+1]. This can be used to define the molar magnetic susceptibility (χm)- the degree of magnetization of a material per mole7.
χm ={NA ge2µ02µb2 S[S+1]}/(3kT) (2.5)
where NA=Avogadro number.
The above expression gives a positive value of susceptibility (χm>0). In result of that,
spin magnetic moment contribute to the other paramagnetic susceptibilities (due to orbital angular momentum) of the material7. However, the thermal motion in the material randomizes the spin orientation, which decreases the intrinsic spin contribution in magnetization. In general, no net spin can be observed in most of the materials due to equal chance of unpaired electrons spinning up and down, classified as diamagnetic or paramagnetic materials according to the reaction of the material in an external magnetic field. The materials with net spin from the domains are considered as ferromagnetic such as iron, cobalt and nickel. In the absence of external magnetic field, the domains of this
12
net spin align in randomized directions. However, in an external magnetic field, align them-self to the magnetic field. The domain alignment of Para and ferromagnetic materials can be depicted in fig.12.
Figure.12 Domain alignment of Para and ferromagnetic materials
The net spin per unit (atom, molecule) in a material defines the magnetic properties. A material shows good magnetic ordering when the energy gained by spin ordering is greater than the energy gained by thermally induced spin disorder. At low temperatures, known as transition temperature, most of the paramagnetic materials undergo phase transition into parallel spins in large number of domains. At this state, the material shows strong magnetism called ferromagnetism and continues with non-zero magnetism even at the absence of the external magnetic field. In case of antiferromagntic materials, the spin orientations in most of the spin domains cancels with each other and no net magnetism remains in the absence of an external magnetic field. The transition temperature for the ferromagnetism is Curie temperature , and for antiferromagnetism is Neel temperature 7.
2.1.1 Spin-orbit coupling
When the nuclei contributions are negligible, the collections of angular moments(s) of all electrons in a material develop total magnetic moment. The angular moment of an electron is further a collection of orbital angular moment and intrinsic angular moment (fig.3). In a simple way, the interaction between the nuclear charge and spin of the electron is called spin-orbit coupling or interaction. In a classical way, it can also explain as, the interaction of magnetic moment of spin and the magnetic field of the orbital angular moment (l>0). However the complete understanding of this interaction is not possible without quantum mechanical explanation10. Practically we can experience spin-orbit interaction by the study of disturbance in the electron cloud due to its own spin. The strength of the coupling and its effect on the energy levels of the atom depends on
the relative orientations of the spin and orbital orbital angular moments 7
Figure.13 Spin-orbit coupling in consequence of spin and orbital angular moments of an electron. Orbital
and intrinsic angular moments (l, s) give rise to magnetic moments (µ).
The magnetic moment is proportional to the angular momentum.
Where γe is magnetogyric ratio of the electron.
γ
e- charge of the electron; m In eqn (2.6), the negative sign indicates angular momentum but proportional to it.
Therefore, an electron possessing orbital angular momentum (l) generates magnetic moment (µr), and spin magnetic moment (µ
of the electron (eqn.3). µr = γel and µ
For spin magnetic moment, the factor increases the magnetic moment twice its expected value.(eqn.2.8)
13
the relative orientations of the spin and orbital magnetic moments, hence (Fig 13).
orbit coupling in consequence of spin and orbital angular moments of an electron. Orbital and intrinsic angular moments (l, s) give rise to magnetic moments (µ).
The magnetic moment is proportional to the angular momentum. µ = γe l
is magnetogyric ratio of the electron.
γe = - e/(2me)
charge of the electron; me- magnetic moment the electron.
), the negative sign indicates that the magnetic moment is anti angular momentum but proportional to it.
electron possessing orbital angular momentum (l) generates magnetic and spin magnetic moment (µs) generates intrinsic angular momentum (s)
l and µs = 2 γes
spin magnetic moment, the factor increases the magnetic moment twice its expected hence on spin and
orbit coupling in consequence of spin and orbital angular moments of an electron. Orbital
(2.6) (2.7) magnetic moment the electron.
the magnetic moment is anti-parallel to the
electron possessing orbital angular momentum (l) generates magnetic intrinsic angular momentum (s)
(2.8)
14
Figure.14 The interaction between spin and orbital magnetic moments results the spin-orbit coupling. The
alignment of angular and magnetic moments develops high (a) and low (b) energy levels. (Adopted from Atkins, Physical chemistry, 8th edition).
When the angular momentum is parallel, then the magnetic moments align unfavorably which gives the high energy level (Fig.14a). On the other hand, anti-parallel alignment of the angular momentum provides favorable interaction of magnetic moments results lower energy level (Fig.14b).
Spin-orbit coupling depends on the nuclear charge. If the nuclear charge is higher, the magnetic field developed by the orbital angular momentum also higher. In addition, the spin-orbit interaction also rises since spin-orbit interaction depends on the interaction between the spin magnetic moment and orbital magnetic field. Along with that, coupling increases sharply with atomic number (Z) 7.
2.1.2 Hyperfine interaction
The nuclei of a molecule or complex produces magnetic field at each electron of that molecule or complex. The interaction between this magnetic field and the electron’s intrinsic moment is called hyperfine interaction. Even though the intrinsic momentum of nucleus is greater than the intrinsic momentum of electron (Fig.15a), magnetic moments of the nuclei acting on the electron are several orders smaller than that of electron intrinsic moment. To understand this, consider the magnetic field of the nucleus and the
electron spin as bar magnets. And and the electron spin are B
magnetic field on the molecule or complex depends o Therefore, the favorable alignment of nuclei with the e generate higher hyperfine interaction and
(a) (b)
Figure.15 (a) The nuclear spin and electron spin in orbital around the nucleus. (b) The interaction of
electron spin and nucleus magnetic field is represented with a bar magnets, where the size of bar magnets depicts the size of magnetic field effect on the electron.(B
There are two major contributions for the hyperfine interaction. and Fermi contact interaction. The dipole
electrons experience field from point magnetic nucleus and p orbital7. This
which is anisotropic (depending
electrons experience Fermi contact interaction
due to spherically distributed electron around the nucleus. of radical orientation). This
character. These two effects are quite large causing hyperfine interaction.
15
electron spin as bar magnets. And also consider the magnetic field produced by nucleus and the electron spin are B1 and B0, respectively. Then, the effect of the external
magnetic field on the molecule or complex depends on the alignment of nuclei (F
Therefore, the favorable alignment of nuclei with the external magnetic field could hyperfine interaction and it could be less for the unfavorable alignment.
(a) (b)
spin and electron spin in orbital around the nucleus. (b) The interaction of electron spin and nucleus magnetic field is represented with a bar magnets, where the size of bar magnets depicts the size of magnetic field effect on the electron.(B0>>B1).
ere are two major contributions for the hyperfine interaction. Dipole
and Fermi contact interaction. The dipole-dipole interaction7 can be found when p orbital electrons experience field from point magnetic-dipole due to the distance b
. This could be well observed in radicals trapped in solids and is anisotropic (depending on the radicals orientation)7. However, the
Fermi contact interaction2 even though no dipole
due to spherically distributed electron around the nucleus. So, It is isotropic (independent of radical orientation). This could be observed in molecules where spin density has s character. These two effects are quite larger when compare with oth
hyperfine interaction.
field produced by nucleus , respectively. Then, the effect of the external n the alignment of nuclei (Fig.15b). rnal magnetic field could less for the unfavorable alignment.
spin and electron spin in orbital around the nucleus. (b) The interaction of electron spin and nucleus magnetic field is represented with a bar magnets, where the size of bar magnets
ipole-dipole interaction can be found when p orbital dipole due to the distance between observed in radicals trapped in solids and . However, the s orbital even though no dipole-dipole interaction It is isotropic (independent in molecules where spin density has s when compare with other contributions
16
2.2 Ferromagnetic half metals
The well known ferromagnetic metals are 3d transition metals e.g., iron, cobalt, nickel etc. The ferromagnetism in 3d transition metal lays in the behavior of 3d and 4s electrons. In solids, 3d and 4s orbital undergoes hybridization which makes both orbitals indistinguishable (Fig.16). However, in metallic state, these two orbital levels broadened into energy bands. The energy band formed by 4s orbital is wide due to larger spread of 4s orbital, and overlap with the other 4s orbital of neighboring atoms. In contrast, the energy band formed by adjacent 3d orbitals is narrow due to less extension of 3d orbital. In the metallic state, even though less mobile than 4s electrons, 3d electrons are the conducting electrons for 3d transition ferromagnetic metals1.
Figure.16 3d and 4s orbital density of states (DOS) of ferromagnetic metals: Fe, Co. (adapted from OSU
sources)
Density of states (DOS) quantifies the arrangement of energy levels in quantum mechanical system, such as energy levels in an atom. It is denoted by the function of internal energy, N (E) which represents the number of the electrons in the given system in between the energy levels E and E+dE 8. The highest occupied energy level is called Fermi energy (EF) level (Fig.16). According to the Pauli Exclusion Principle, the valance
orbital filled with higher density of spin up (↑) electrons than spin down (↓) electrons is a reason for the resultant magnetization. This can be understand by spin polarization (P) which is the ratio of difference in the number of spin-up and spin-down electrons to the total number of electrons (eqn.2.9). For paramagnetic materials P=0. But, for ferromagnetic metals like Fe and Co, the electrons with spin up (↑) are more (Fig.16) .i.e., P>0.
17
The ferromagnetic materials with only one spin of conduction electrons (either spin up (↑) or spin down (↓)) are called ferromagnetic half metals. As a consequence, 100% spin polarization (P=1) occurs in this material, where the minority spin electrons behaves like semi conducting and majority spin electrons like normal metallic in nature9.The fist known half metallic ferromagnet is CrO2 [4, 5]. The density states of CrO2 contains only
similar-spin electrons occupied spin polarized sub-band at Fermi-level (Fig.17), indistinct to ferromagnetic metals like Fe and Co , where the valance density of states filled with spin polarized 3d electrons and unpolarized 4s electrons11.
Figure.17 The density of states (DOS) of ferromagnetic half-metal, CrO2.filled with only spin up
electrons, represents 100% spin-polarization. (From OSU, Department of physics)
The ferromagnetic material should provide 100% spin-polarization to consider as half-metal. But the actual situation is different when it comes to applications. Spin-polarization at surface and at interface with other medium has greater priority. The surface and interface electronic structures are in general different than the bulk electronic structure. In case of CrO2, the half-metallic properties are almost stable at surface, as well
as at interface with other medium. The rutile structure11 like crystallization indeed allows CrO2 in stoichiometric 11,12. This structure prevents reconstruction and surface
segregation12. In consequence of that, CrO2 shows highest degree of spin-polarization.
Identifying metallic or highly spin–polarized materials has significantly improved possibility of implementing magneto-electronics (spintronics) in device fabrication. Intense research has been evaluating half-metallicity in many materials like perovskites13, showing major interest to use in spintronics. Experimental and theoretical predictions of the spin-polarized La0.7Sr0.3MnO3(LSMO) with unusual electronic, structural and
magnetic properties that are attractive to utilize in spintronic devices. In perovskite oxides spin-polarization varies from 35% to 100%13.The difference between the Fermi level and minority carrier band is higher in LSMO than CrO2 (Fig.18). Moreover,
Perovskite structures in contrast to CrO2 structure, are not stoichiometric surfaces.
spin-polarization value of LSMO var on interface medium, La0.7
Figure.18 The density of states (DOS), N (E) of LSMO. The Fermi level falls below a certain energy
gap (δ) in the total energy gap ∆
18
polarization value of LSMO varies with the interface medium. And thus,
0.7Sr0.3MnO3 (LSMO) shows different magnetoresistance
The density of states (DOS), N (E) of LSMO. The Fermi level falls below a certain energy ) in the total energy gap ∆.
And thus, Depending magnetoresistance values.
19
2.3 Magnetoresistance
For characterization of spintronic devices, it is important to understand the difference between different magnetoresistances to realize the real effect in the device. A simple definition for the magnetoresistance is the change in electrical resistance under the influence of an external magnetic field . It was first observed by Lord Kelvin in 1856.He observed a 0.2% increase in resistance (R) of iron when applying the magnetic field along the direction of the current and decrease of 0.4% for the transverse applied field named as Anisotropic magnetoresistance (AMR)14. Anisotropic magnetoresistance (AMR) is the first known phenomenon in which the difference in magnetoresistance observed when the electric current applied between parallel or perpendicular to the magnetization as explained above14. This effect is a feature of metallic ferro magnets and is usually due to the spin-orbit coupling accommodating a mixing of minority or majority spin states15. This effect is very small compare to other magnetoresistive effects and typically in the order of 1 or 2%.
Some other Magnetoresistance effects discussed here are Giant Magnetoresistance (GMR), Tunnel Magnetoresistance (TMR), Colossal Magnetoresistance and Organic magnetoresistance since it is important to understand difference and primary reasons for the magnetoresistance effect.
2.3.1 Giant magnetoresistance (GMR)
It is a large change in electrical resistance with an applied magnetic field. Initially, GMR devices are simple spin valve structure of two ferromagnetic (FM) layers of same coercive field, separated by a thin layer of semi-conducting material shown in Fig.19. GMR is caused by the spin dependent scattering of electrons between two FM layers. In the absence of an external magnetic field, random alignment of magnetic domains in FM layers increase scattering of electrons, resulting in high resistance to the current passing through one FM to the other. However, parallel magnetization of two FM layers, decreases resistance with the decrease of spin dependent scattering. According to the definition of magnetoresistance1, the above effect gives negative Giant magnetoresistance (negative GMR).
(a)
Figure.19 A-semiconductor layer, B
magnetic field. (a) In the absence of external magnetic field, random alignment of Two B layers gives higher C. (b) In presence of external magnetic field, parallel alignment
Recently, GMR effect is improved coercive fields1(Fig.20), b
The magnetization of the FM magnetic moments of the two
of external magnetic field, the electrons with parall therefore decreases the total resistance.
moments at low fields increases the resistance (Fig.20).
(a)
Figure.20 (a) Anti-parallel magnetization of two
scattering at both layers. (b) Parallel magnetization of two resistance.
20
(b)
semiconductor layer, B-ferromagnetic material, C-resistance to the current, D
(a) In the absence of external magnetic field, random alignment of Two B layers gives presence of external magnetic field, parallel alignment of FM layers decreases resistance.
is improved and well controlled by using FM layers of d
), by changing the magnetic fields in-between the coercive fields. magnetization of the FM’s can be tuned between parallel and anti
magnetic moments of the two ferromagnetic (FM) layers are parallel under the influence of external magnetic field, the electrons with parallel spin undergo
total resistance. And the anti-parallel alignment of moments at low fields increases the spin dependent scattering, and hence
(b)
parallel magnetization of two FM layers increases resistance by spin dependent scattering at both layers. (b) Parallel magnetization of two FM layers provides sp
resistance to the current, D-External (a) In the absence of external magnetic field, random alignment of Two B layers gives
layers decreases resistance.
and well controlled by using FM layers of different the coercive fields. anti-parallel. If the layers are parallel under the influence ndergoes less scattering, parallel alignment of magnetic hence increases the
FM layers increases resistance by spin dependent FM layers provides spin up electrons less
21
2.3.2 Tunnel magnetoresistance (TMR)
The applied voltage on a very thin insulator sandwiched between two ferromagnetic contacts, generates tunnel current due to quantum effect17( Fig.21). Here the insulating layer should only be few atomic layers of thick, providing quantum mechanical tunneling of electrons from one ferromagnet to other. The electrical resistance will be low when the magnetizations of two ferromagnetic layers are parallel. On the other hand, the electrical resistance rises for anti parallel magnetization of the ferromagnetic layers is known as the Tunnel magnetoresistace effect (TMR)18. However; TMR is a pure interface effect and
does not require spin transportation in insulator layer.
Figure.21 The current (i) developed by quantum mechanical tunneling of electrons from one ferromagnet
to the other through semiconductor insulator. Under external magnetic field, it shows tunnel magnetoresistance effect.
2.3.3 Colossal magnetoresistance
In certain manganese oxides, huge resistance change is observed under an external magnetic field which cannot be compared with any other magneto resistance effects observed, and is named as the colossal magnetoresistace (CMR). In contrast to the GMR materials which require very low external magnetic field (few Oersteds), very large external magnetic fields (several Teslas) are required for the CMR materials. For instance, a perovskite manganite, LMnO3 consists of large lanthanum cat-ion at the center
of the unit structure ( Fig.22) surrounded by oxygen (small in size) and manganese (medium in size) ions. Following Hund’s rule, four electrons in manganese have the same spin state occupying lower triplet state by three electrons and one electron in a doublet state10 (Fig.22b).
(a) (b)
Figure.22 (a) Unit structure of LMnO
If a fraction of La3+ ions are replaced by Sr
xSrxMnO3, then the resistance drops dramatically and the system becomes ferromagnetic
with a curie temperature around room temperature the material is insulating and non
ferromagnetic at below Tc
resistive effect called colossal ion on La site forces Mn
neighboring Mn sites creates the possibility of electron hopping between these two sites via intervening O2- ions known
is spin polarized, according to Hund’s rule, remembers its spin state to Mn
neighboring Mn3+ and Mn cos(θ/2) where θ is the angle b
2.3.4 OMAR (Organic magnetoresistance)
The magnetoresistance up to ~10% observed
ferromagnetic electrodes (no spin coherent charge injection
spin valve effect20. It was known from the previous work that universal in organic devices,
the primary reason for the OMAR
models are proposed to explain the OMAR effect. Th singlet-triplet inter-conversion model
and 3) The bipolaron model
pair and subsequent exiton formation the bipolaron model is a single carrier model.
22
(a) (b)
nit structure of LMnO3. (b) Valance electronic structure of Mn.
ions are replaced by Sr2+, Ca2+ or Ba2+ ions in LaMnO
, then the resistance drops dramatically and the system becomes ferromagnetic with a curie temperature around room temperature19. Above Tc (transition temperature)
the material is insulating and non-magnetic, on the other hand it is metallic and
c. In particular, near Tc the material shows very large
olossal magnetoresistance (CMR). By substituting 2+ ion ion on La site forces Mn3+ ionic state to Mn4+ state. Whereas Mn3+
neighboring Mn sites creates the possibility of electron hopping between these two sites known as double exchange10. The current developed by hopping according to Hund’s rule, the electron that hops away from Mn remembers its spin state to Mn4+. However, this is only possible when the net spins of the
and Mn4+ are in the same direction, following of angle dependence to is the angle between their spin directions10,19.
MAR (Organic magnetoresistance)
The magnetoresistance up to ~10% observed in a simple organic diode structures electrodes (no spin coherent charge injection or detection
. It was known from the previous work that the universal in organic devices, named as Organic magnetoresistance (OMAR)
primary reason for the OMAR effect is still under debate. At present, t
proposed to explain the OMAR effect. They are 1) Magnetic field induced conversion model21, 2) The triplet exiton-polaron quenching mo on model23. The first two models are based on singlet or triplet charge pair and subsequent exiton formation, i.e. a two carrier process. While the
bipolaron model is a single carrier model.
ions in LaMnO3 into La
1-, then the resistance drops dramatically and the system becomes ferromagnetic sition temperature) magnetic, on the other hand it is metallic and the material shows very large magneto (CMR). By substituting 2+ ion with 3+
3+
and Mn4+ are the neighboring Mn sites creates the possibility of electron hopping between these two sites . The current developed by hopping the electron that hops away from Mn3+ . However, this is only possible when the net spins of the
me direction, following of angle dependence to
diode structures without or detection) is not due to the the OMAR effect is named as Organic magnetoresistance (OMAR)2. However, At present, three different agnetic field induced polaron quenching model22
on singlet or triplet charge i.e. a two carrier process. While the third model,
Prigodin et al.21 has proposed model with the basic idea limited. In this process annihilation with different rate
charge carriers. It is shown from eqn.1 that the space increases with decreasing e
depends on the degree of mixing between singlet(S) and triplet (T) states. In this model, it is argued that the magnetic field disturbs the degeneracy of triplet states and allows only anti-parallel spin state (T0
total e-h recombination rate (Fig.23).
Here V is the voltage drop across the semiconductor thickness L. of electrons and holes respectively, and
the relative dielectric constant of the semiconductor.
Figure.23 Singlet-triplet interconversion for strong magnetic field.
The e-h recombination process goes through a state corresponding formation r
dissociates back to free charge carriers. The recombination rate spin state of e-h pair, with rates k
and triplet pairs may also dissociate back to free charge carriers with rate constants q qt respectively. The e-h recombination
h formation rate constant ( triplets(Fig.24).
23
proposed the magnetic field induced singlet-triplet inter with the basic idea that the OMAR effect is electron-hole (e
his process there is a formation of correlated e-h pair ifferent rates for singlet and triplet pairs or dissociate
charge carriers. It is shown from eqn.1 that the space-charge limited current density (J) increases with decreasing e-h recombination rate. The e-h recombination rate (
degree of mixing between singlet(S) and triplet (T) states. In this model, it is argued that the magnetic field disturbs the degeneracy of triplet states and allows only
0) to mix with the singlet (S) state, which further
rate, and hence increases the space-charge-limited current µ µ µ µ
Here V is the voltage drop across the semiconductor thickness L. µ, µ
of electrons and holes respectively, and is the dielectric constant of vacuum and constant of the semiconductor.
triplet interconversion for strong magnetic field.
n process goes through a state of e-h pair, depends on the corresponding formation rate constant (b). Then the e-h pair either recombines or
back to free charge carriers. The recombination rate further with rates ks and kt for singlet and triplet pairs
rs may also dissociate back to free charge carriers with rate constants q h recombination rate can be defined with an empirical formula constant (b), individual formation and dissociation rates of singles
triplet inter-conversion hole (e-h) recombination h pair and subsequent d triplet pairs or dissociate back into free charge limited current density (J) h recombination rate ( further degree of mixing between singlet(S) and triplet (T) states. In this model, it is argued that the magnetic field disturbs the degeneracy of triplet states and allows only further decreases the limited current density
(2.10)
µ are the mobility
is the dielectric constant of vacuum and -
h pair, depends on the h pair either recombines or further depends on the for singlet and triplet pairs21,22. These singlet rs may also dissociate back to free charge carriers with rate constants qs and
be defined with an empirical formula of e-individual formation and dissociation rates of singles and
Figure.24 Schematic illustration of (i) F
subsequent, (ii) dissociation or (iii) recombination.
P. Desai et, al.22 proposed the triplet
effect is due to the trapping of charge carriers
polymer. This model is further concluded by the study of photocurrent by the recombination of exitons,
with the singlet-triplet interconversion model. contribution of magnetoresistance comes from of a free carrier and the triplet
The model also explained the phenomena of singlet detailed manner. The magnetic field induced singlet reason to increase the rate of intersystem crossing (k
depend on the relative concentration of singlets and triplets as well as the temperature of the system. If there is an excess of triplets and the temperature is high enough to overcome the energy barrier, then increase in k
concentration, i.e excessive triplets transfer into singles. singlet concentration, then increase in k
concentration22. The entire process
with an excitation pump (P) that can genera either singlet or triplet, and intersys
24 !" #$ #$%$& " #' #'%'
illustration of (i) Formation of intermediate electron-hole pairs and then the subsequent, (ii) dissociation or (iii) recombination.
proposed the triplet exiton-polaron quenching model,
effect is due to the trapping of charge carriers (polarons) at triplet exiton states
This model is further concluded by the study of photocurrent by the and at below turn-on voltage of the device,
triplet interconversion model. But above turn on voltage, the main contribution of magnetoresistance comes from the magnetic field dependent
a free carrier and the triplet exiton.
also explained the phenomena of singlet-triplet interconversion in manner. The magnetic field induced singlet-triplet interconversion
increase the rate of intersystem crossing (kISC). The increase in k
depend on the relative concentration of singlets and triplets as well as the temperature of the system. If there is an excess of triplets and the temperature is high enough to overcome the energy barrier, then increase in kISC would lead to decrease
excessive triplets transfer into singles. However, if there is an excess of singlet concentration, then increase in kISC would lead to increase in triplet
. The entire process can be seen in a simple schemati with an excitation pump (P) that can generate either singlets or triplets,
, and intersystem crossing between two states22.
(2.11)
hole pairs and then the
polaron quenching model, predicts that the polarons) at triplet exiton states of the This model is further concluded by the study of photocurrent by the , this model agrees But above turn on voltage, the main etic field dependent interaction
triplet interconversion in a more triplet interconversion is the possible ). The increase in kISC would
depend on the relative concentration of singlets and triplets as well as the temperature of the system. If there is an excess of triplets and the temperature is high enough to lead to decrease in triplet , if there is an excess of would lead to increase in triplet schematic diagram (Fig.25) te either singlets or triplets, recombination of
Figure.25 A schematic diagram of the excitation and recombination pathways in an organic
molecule.
The bipolaron model is proposed by W. Wagemans et al. two models based on spin
of the OMAR effect with the partial blocking of bipolarons at relatively high magnetic fields. This model has been investigated via Monte Carlo simulations a
two-site model and suggested experimental study.
transportation of charge carriers under the influence of an applied magnetic field is studied.
The charge transportation
hopping (VRH), through spatially and energetically rand bipolaron formation in this
same site with anti-parallel spin alignment. Two carriers occupy different characteristic sites, α and β with parallel (P) or anti
bipolaron forms when there is an anti
site α to β. Otherwise in parallel alignment, the transfer is not possible and occurs. Instead of going t
environment (e). The ratio of these two rates is defined as the branching ratio (b).
In reality, the carriers in organic materials experience random hyperfine field from the surrounding hydrogen atoms
an external magnetic field,
increases the probability of forming a bipolaron. magnetic field, when it is
25
schematic diagram of the excitation and recombination pathways in an organic
proposed by W. Wagemans et al.23which contradict spin-dependent exitonic effects. It explains the qualitativ
with the partial blocking of bipolarons at relatively high magnetic fields. This model has been investigated via Monte Carlo simulations a
site model and suggested experimental study. In both of these anal
transportation of charge carriers under the influence of an applied magnetic field is
charge transportation in most of the organic semiconductors is by variable range hopping (VRH), through spatially and energetically randomized, localized sites bipolaron formation in this type of system is only possible when two carriers occupy
parallel spin alignment. Two carriers occupy different characteristic with parallel (P) or anti-parallel (AP) spin alignment (Fig.26
when there is an anti-parallel spin alignment with charge received from in parallel alignment, the transfer is not possible and
of going to site β, the carrier from α hop to the surrounding sites in the environment (e). The ratio of these two rates is defined as the branching ratio (b).
In reality, the carriers in organic materials experience random hyperfine field from the atoms, which is different in each hopping sites. In the absence of external magnetic field, the hyperfine field randomizes the spin orientation and increases the probability of forming a bipolaron. However, in the presence of an external when it is larger than the hyperfine interaction, the carriers align more
schematic diagram of the excitation and recombination pathways in an organic
which contradict with the other explains the qualitative behavior with the partial blocking of bipolarons at relatively high magnetic fields. This model has been investigated via Monte Carlo simulations and an analytical analytical models, the transportation of charge carriers under the influence of an applied magnetic field is
in most of the organic semiconductors is by variable range omized, localized sites2. The system is only possible when two carriers occupy parallel spin alignment. Two carriers occupy different characteristic (AP) spin alignment (Fig.26). So the with charge received from in parallel alignment, the transfer is not possible and spin blocking23
to the surrounding sites in the environment (e). The ratio of these two rates is defined as the branching ratio (b).
In reality, the carriers in organic materials experience random hyperfine field from the which is different in each hopping sites. In the absence of hyperfine field randomizes the spin orientation and n the presence of an external larger than the hyperfine interaction, the carriers align more
